Voltage regulators are circuits designed to maintain a constant voltage output regardless of input or load conditions.
FIG. 1 is a diagram illustrating a conventional regulator. The regulator 100 compares a reference voltage 102 with a feedback voltage from the output 108 in a transconductance amplifier 104 to produce a current which is then multiplied by a current mirror 106 to yield a current which drives the output 108.
Clock switching places large load transients on such regulators, too quickly for them to respond. For example, on a clock switch, current demand can go from 75 μA to 75 mA in 4 ns. With a 15 nF load, this causes a supply change of 5 mV/ns. However, a typical regulator will only have around 100 mV margin at best.
While external capacitors can be used to provide enough charge to give the regulators time, but due to parasitics, only for relatively small transients. Further, in an integrated circuit environment, an external capacitor costs a precious pin while inviting assembly problems with regulatory consequences.